Tungsten metal products are made using a chemically refined intermediate ammonium paratungstate (APT) product. APT is produced by processing tungsten concentrates containing scheelite (calcium tungstate) or wolframite (iron/manganese tungstate). The processing steps usually include several impurity removal steps to make an APT product of the desired purity.
Commercially, two general approaches typically are being used for APT production: (1) digestion in basic solution, and (2) digestion in strong acids. In the basic leaching processes, tungsten concentrates are digested with caustic and/or soda ash solutions. The resulting solution is subjected to a series of purification stages for P, As, and Mo removal. The purified Na.sub.2 WO.sub.4 solution is then converted into APT by solvent extraction and crystallization. In the acid leaching processes, tungsten concentrates are leached with strong hydrochloric acid which solubilizes impurities and precipitates crude tungstic acid. The resulting tungstic acid is washed and dissolved in ammonia solution prior to crystallization of APT. The APT product is then thermally decomposed and reduced to produce tungsten metal. The purity of the tungsten metal is directly proportional to the purity of the APT from which it is made.
The final step in production of APT using either of the above methods is crystallization of APT from ammonium tungstate solution. The crystallization step is an important part of the process, since many impurities concentrate in the crystallizer mother liquor and do not crystallize with the APT.
Both processing methods can produce 99.95 to 99.99 percent APT. This purity of APT has been used in the production of tungsten carbide, tungsten-containing catalyst, ferrotungsten and tungsten filament. Because of technological advances, high purity tungsten is in demand in various catalyst, electric, and electronic applications. In order to up-grade three-nine or four-nine purity APT, redissolution and recrystallization has been used.
Chemically, the crystallized APT is not readily soluble in ammonia solutions. In order to up-grade an APT product, the APT has to be either (1) processed through caustic dissolution, impurity removal, solvent extraction and recrystallization stages, or (2) calcined at 350.degree. to 600.degree. C. to form yellow oxide. The more soluble yellow oxide is then dissolved in ammonia solution prior to recrystallization of APT. Sometimes, several processing cycles are performed in order to obtain five-nine and six-nine APT products. These approaches are labor and cost intensive, and also tungsten yield may be low.
In a tungsten facility that produces APT, there are several stages in the operation where APT may form that is not the proper particle size or purity to be included in the normal production. When APT is made in a continuous crystallizer, some of the crystallized product is too fine to include in the production material that is sold. This material is packaged separately from product and is labeled as dust. Dust typically must be recycled. Also, in normal operations, some APT product may be made that does not meet required purity specifications. This APT may be sold at a discount, or if it can not be sold it must be recycled in the process. Each of these products contributes to recycle in a tungsten operation that increases operating costs. If the APT-containing solids could be redissolved in ammonia, recycled material could be inserted at an intermediate point in the process rather than at the head end, costs would be decreased, and overall output would increase.
In the present invention, conditions with regard to temperature and ammonia concentrations for direct dissolution of APT have been defined. The advantages of the invention include:
1. Recycle of off-grade APT or APT fines is minimized by direct dissolution and recrystallization in ammonia solution, and PA1 2. Dissolution and recrystallization of off-grade or normal APT product yields a purer product in a simple purification process. High-purity APT can be produced with fewer processing steps.